1. Field of the Invention
This invention relates to a photographic process in which an image-wise exposed silver halide photographic element is developed so that the image produced is enhanced by the presence of a dye.
2. Background of the Prior Art
To reduce the cost of photographic films, it is necessary to reduce the silver content thereof while maintaining the sensitivity of the film and the image density obtained upon processing.
In the conventional black and white photographic process, silver halide which has been image-wise exposed to light is chemically reduced by a developer solution to give an image composed of metallic silver in the exposed areas. In color photography the exposed silver halide is reduced by a developing agent which, in its oxidized form, is capable of reacting with certain organic compounds to form dye images. These compounds, which may be present in the coated photographic element or in the developer solution, are known as color couplers. In color photography the silver image is produced as a by-product and is removed by oxidation in a subsequent bleach or bleach-fix bath.
In such a color development process the quantity of dye formed in the image areas is strictly governed by the quantity of silver halide which is reduced by the developing agent. Either two or four atoms of silver are required to produce one molecule of dye, depending upon the particular type of color coupler.
Various methods have been proposed in which the ratio of the dye formed to the silver required is increased, thereby reducing the quantity of silver required to produce an image density comparable to that in the conventional photographic process. The silver image formed by conventional development may be employed as a catalyst for the oxidation of a color developing agent by an oxidant, referred to as an "amplifying agent". In this way, many molecules of a dye may be formed for each atom of silver present.
There are several known photographic processes employing amplifying agents including cobalt (III) hexa-amine salts, chlorites, aryl iodoso compounds and hydrogen peroxide. The amplifying agent having the highest activity is hydrogen peroxide, although there are several disadvantages with this compound which have prevented its use in commercial photographic processes.
British Patent Specification No. 1 268 126 discloses a process in which a silver halide film which contains an excess of color coupler is developed in a standard color developer solution. The film is then transferred to a bath containing aqueous hydrogen peroxide in which the color developing agent is oxidized by the hydrogen peroxide in those areas where developed silver halide is present to catalyze the reaction. The oxidized developer then reacts with color coupler to form a dye enhanced image. One problem with this process is that the undeveloped silver halide in the non-image areas also catalyzes the oxidation of color developer by hydrogen peroxide, resulting in a high background fog. U.S. Pat. Nos. 4,045,225, 4,045,226, 4,069,050, 4,062,684 and 4,113,490 disclose possible solutions to this problem in which compounds which deactivate the undeveloped silver halide, thereby reducing the fog, may be added to the coated layer.
A further disadvantage with the hydrogen peroxide amplification process is that the catalytic action of the developed silver may be very easily poisoned by a variety of substances including thiosulfate fixer and iodide ion. Also, large grained silver images such as those produced by medical X-ray film do not behave as very active catalysts for amplification. The activity of the silver image as a catalyst appears to be inversely related to the particle size of the silver.
U.S. Pat. No. 4,088,486 discloses a photographic process in which the silver image is replaced by a cobalt (II) image. The cobalt (II) acts as a catalyst for hydrogen peroxide amplification and is not subject to poisoning by thiosulfate fixer. Accordingly, the developed film may be fixed prior to amplification thereby reducing the background fog caused by silver halide in the non-image areas. Furthermore, the cobalt (II) image is not subject to grain size effects on its catalytic activity. However, a disadvantage of this cobalt amplification process is that it may not generally be used in combination with soluble color couplers since these compounds tend to react with the cobalt catalyst. Various other methods of enhancing the image density of silver halide images which involve initial bleaching of the silver image are known. In one process the bleached image is simply redeveloped by a black and white developer furnishing a slightly enhanced image. In another process the bleached image may be treated with a solution which causes the image-wise deposition of a pigment in a double decomposition reaction. In a further process a color photographic material is color developed, bleached and then color developed again thereby yielding twice the normal quantity of dye. All of these methods give a final image density which is strictly governed by the quantity of silver present since the reactions involved are completely stoichiometric.
It is an aspect of the present invention to provide a photographic process in which silver images may be enhanced by the formation of a dye without the significant problems attendant in the prior art.